Torsional loads on drill strings can reduce the axial compressive load limit for buckling the drill string. The larger the torsional load, the lower the buckling load. This paper presents a new analysis of drill-string buckling under axial compressive and torsional loads. New equations for buckling loads and lengths are derived for drill strings in vertical, inclined, horizontal, and curved wellbores.

The new analysis includes a model of the work done by the torsional load in the buckling process and provides a better understanding of drill-string buckling under the combined compressive and torsional loads. The results also agree with a previous study for horizontal wellbores.

With the new equations, drill-string buckling, drag, and torque can be predicted more easily and accurately for drilling horizontal wells. An example calculation is provided to illustrate the use of the equations for predicting drill-string buckling while rotary-drilling horizontal wells.

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